core: Kill usbi_os_backend structure definition madness

core: Kill usbi_os_backend structure definition madness

Prior to this commit, each backend provided its own uniquely named
usbi_os_backend structure and a massive pile of #ifdefs assigned the
global usbi_backend pointer to the correct one. This commit kills off
all this code and instead has each backend provide the usbi_backend
symbol directly. The linker can inform of any issues that might arise
with symbols.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>